1. Field of the Invention
The present invention relates to an automatic sheet feeding apparatus for feeding a recording sheet to a recording system or a copying machine, and for successively feeding originals to an image reading system and the like.
2. Related Background Art
In the past, as automatic sheet feeding apparatus for successively feeding stacked recording sheets or originals (referred to generically as "sheets" hereinafter) one by one to a recording system or an image reading system, the following three types have been well used.
A sheet feeding apparatus of the first type, which has been most widely used, is of the claw separation type, an example of which is shown in FIG. 28. This apparatus comprises a pair of substantially triangular separating claws 101 associated the front corners of stacked sheets 100, a pair of sheet supply rollers 102 (only one of which is shown) abutted against an upper surface of the sheet stack and adapted to feed the sheets 100 downwardly, a sheet supply roller shaft 103, a pressure plate 104 for urging the sheet stack 100 against the sheet supply rollers 102, a spring (not shown) for biasing the pressure plate 104 upwardly, separation sheets 105 adapted to prevent double-feed of the sheets 100 and adhered to the pressure plate 104 in confronting relation to the sheet supply rollers 102, and a sheet cassette 106. This apparatus is designed so that the sheets are separated one by one by creating a loop in the sheet by making good use of the resilience of the sheet (resistance to the bending of the sheet) to ride the sheet over the separating claws 101.
A sheet feeding apparatus of the second type is of the friction separation type, an example of which is shown in FIG. 29. This apparatus comprises a separation roller 102 having a relatively large diameter, a separation roller shaft 103, a separation pad 107 urged against the separation roller 102, a spring (not shown) for biasing the separation pad 107 upwardly, a feed roller 108 for feeding sheets 100 intermittently, and a pinch roller 109. This apparatus is designed so that the sheets are separated by using a friction force.
A sheet feeding apparatus of the third type is of the so-called bank separation type, an example of which is shown in FIG. 30. In this apparatus, a bank 110 having a ramp surface included at an appropriate angle is disposed at a downstream side of a sheet stack 100 in a sheet feeding direction, and the sheets 100 are separated one by one by bending the top sheet 100a along the ramp surface of the bank 110 by a feeding force of sheet supply rollers 102 against which the sheet stack 100 is urged by a pressure plate 104 and a pressure plate spring 111.
However, the above-mentioned sheet separating techniques have the following drawbacks.
First, in the sheet feeding apparatus of the claw separation type, the sheets are separated by bending the front corner portions of the sheet to form a loop by the separating claws 101. Thus, it is difficult to form the loop, and, thus, to separate thicker sheets such as post cards, envelopes and the like which are hard to be bent. Further, relatively thin sheets have less resilience. Thus, loops are formed on a plurality of sheets simultaneously and, thus, it is difficult to separate the sheets one by one. Further, even for sheets having a normal thickness, under high temperature and high humidity circumstances, the sheets absorb moisture to weaken their resilience, and two or more sheets can be separated simultaneously, similar to the aforementioned drawback as to thin sheets.
In addition, a feeding force of the sheet supply rollers 102 must be increased to form the loop in the sheet. Since the separation pads 105 are formed on the pressure plate 104 under the sheet stack 100 to generate a predetermined friction force for preventing double-feed of the sheets (the last two sheets are fed simultaneously), when no sheet is present, the sheet supply rollers 102 are slidingly contacted with the separation pads 105 directly, thereby increasing the load. Thus, a motor having greater torque is required. Further, since a space for accommodating the loop of the sheet is required above the separating claws 101, the size of apparatus becomes large.
In the sheet feeding apparatus of the friction separation type, the sheets are separated by the squeezing action of the friction pad 107 and the separation roller 102. Thus, the apparatus has greater freedom as to the range of thickness of the sheet to be used, for example, from normal copying sheets to post cards. However, the ability for separating thin sheets is unstable, and the diameter of the separation roller 102 must be relatively large, thus the size of the apparatus must be increased. Further, the lower sheet cannot be separated unless the separation pad 107 is always contacted with the separation roller 102. Thus, even after a sheet is fed, the separation roller 102 must be rotatingly driven, with the result that the separation roller 102 is always subjected to a rotational load. Thus, a motor having greater torque is required.
Further, in a printing system, when a trailing edge of the sheet leaves the separation roller, the variation in load causes uneven feeding of the sheet, thereby decreasing the image quality. In addition, the sheets are separated at their leading edge portions. Thus, an additional means is required for abutting the sheet 100 against a nip between the separation roller 102 and the separation pad 107, and, therefore, the feeding means 108, 109 must be arranged at an upstream side or another pad capable of being contacted with and separated from the separation roller 102 must be arranged, thus making the apparatus complicated, large and expensive.
In the sheet feeding apparatus of the bank separation type, although the construction thereof is relatively simple, since the resilience of the sheet is utilized to separate the sheets, it is difficult or impossible to separate sheets other than post cards and other sheets having the same thickness as that of a post card (having the same resilience as that of a post card); therefore, such bank separation technique cannot be used with sheet feeding apparatuses other than an automatic sheet feeding apparatus for thicker sheets such as a post card feeder.
In this way, none of the above-mentioned three separation types can reliably separate various sheets from thin sheets to thicker sheets such as post cards, envelopes and the like, and, thus, the permissible kinds of sheets to be separated are limited. Accordingly, in order to treat sheets having various thickness, since the above-mentioned three separation types must be combined and be switched to treat the respective sheets, or the sheet feeding apparatus must be replaced to cope with sheets having a specific thickness, not only the operability is deceased but also the system becomes expensive and large.